Al capping layers for nondestructive x-ray photoelectron spectroscopy analyses oftransition-metal nitride thin filmsGrzegorz Greczynski, Ivan Petrov, J. E. Greene, and Lars Hultman Citation: Journal of Vacuum Science & Technology A 33, 05E101 (2015); doi: 10.1116/1.4916239 View online: http://dx.doi.org/10.1116/1.4916239 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/33/5?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in X-ray Photoelectron Spectroscopy Analyses of the Electronic Structure of Polycrystalline Ti1-xAlxN Thin Filmswith 0 ≤ x ≤ 0.96 Surf. Sci. Spectra 21, 35 (2014); 10.1116/11.20140506 Erratum: “X-ray photoelectron spectroscopy study of irradiation-induced amorphization of Gd 2 Ti 2 O 7 ” [Appl.Phy. Lett. 79, 1989 (2001)] Appl. Phys. Lett. 80, 3650 (2002); 10.1063/1.1472474 X-ray photoelectron spectroscopy study of irradiation-induced amorphizaton of Gd 2 Ti 2 O 7 Appl. Phys. Lett. 79, 1989 (2001); 10.1063/1.1402647 In situ X-ray Photoelectron, Ultraviolet Photoelectron, and Auger Electron Spectroscopy Spectra from First-RowTransition-Metal Nitrides: ScN, TiN, VN, and CrN Surf. Sci. Spectra 7, 167 (2000); 10.1116/1.1360984 X-ray photoelectron spectroscopy analyses of metal stacks etched in Cl 2 / BCl 3 high density plasmas J. Vac. Sci. Technol. B 16, 147 (1998); 10.1116/1.589770

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.236.172.146 On: Mon, 30 Mar 2015 14:51:57

Al capping layers for nondestructive x-ray photoelectron spectroscopyanalyses of transition-metal nitride thin films

Grzegorz Greczynskia)

Thin Film Physics Division, Department of Physics (IFM), Link€oping University, SE-581 83 Link€oping,Sweden

Ivan PetrovThin Film Physics Division, Department of Physics (IFM), Link€oping University, SE-581 83 Link€oping,Sweden and Materials Science Department and Frederick Seitz Materials Research Laboratory,University of Illinois, Urbana, Illinois 61801

J. E. GreeneThin Film Physics Division, Department of Physics (IFM), Link€oping University, SE-581 83 Link€oping,Sweden; Materials Science Department and Frederick Seitz Materials Research Laboratory,University of Illinois, Urbana, Illinois 61801; and Department of Physics, University of Illinois,Urbana, Illinois 61801

Lars HultmanThin Film Physics Division, Department of Physics (IFM), Link€oping University, SE-581 83 Link€oping,Sweden

(Received 28 January 2015; accepted 16 March 2015; published 30 March 2015)

X-ray photoelectron spectroscopy (XPS) compositional analyses of materials that have been air

exposed typically require ion etching in order to remove contaminated surface layers. However,

the etching step can lead to changes in sample surface and near-surface compositions due to

preferential elemental sputter ejection and forward recoil implantation; this is a particular problem

for metal/gas compounds and alloys such as nitrides and oxides. Here, the authors use TiN as a

model system and compare XPS analysis results from three sets of polycrystalline TiN/Si(001)

films deposited by reactive magnetron sputtering in a separate vacuum chamber. The films are

either (1) air-exposed for �10 min prior to insertion into the ultrahigh-vacuum (UHV) XPS system;

(2) air-exposed and subject to ion etching, using different ion energies and beam incidence angles,

in the XPS chamber prior to analysis; or (3) Al-capped in-situ in the deposition system prior to

air-exposure and loading into the XPS instrument. The authors show that thin, 1.5–6.0 nm, Al

capping layers provide effective barriers to oxidation and contamination of TiN surfaces, thus

allowing nondestructive acquisition of high-resolution core-level spectra representative of clean

samples, and, hence, correct bonding assignments. The Ti 2p and N 1s satellite features, which are

sensitive to ion bombardment, exhibit high intensities comparable to those obtained from

single-crystal TiN/MgO(001) films grown and analyzed in-situ in a UHV XPS system and there is

no indication of Al/TiN interfacial reactions. XPS-determined N/Ti concentrations acquired from

Al/TiN samples agree very well with Rutherford backscattering and elastic recoil analysis results

while ion-etched air-exposed samples exhibit strong N loss due to preferential resputtering. The

intensities and shapes of the Ti 2p and N 1s core level signals from Al/TiN/Si(001) samples do not

change following long-term (up to 70 days) exposure to ambient conditions, indicating that the thin

Al capping layers provide stable surface passivation without spallation. VC 2015 American VacuumSociety. [http://dx.doi.org/10.1116/1.4916239]

I. INTRODUCTION

Refractory ceramic transition-metal (TM) nitride thin

films grown by physical vapor deposition attract increasing

scientific and technological interest due to their unique prop-

erties combining high hardness,1–4 good high-temperature

oxidation resistance,5–7 electrical conductivity ranging from

metallic to semiconducting,8,9 superconductivity,9–11 and

optical absorption, which can be tuned across the visible

spectrum.8 Applications include wear-resistant coatings on

high-speed cutting tools12,13 and engine components,14,15

diffusion barriers in electronic devices,16–20 and bioimplant

coatings.21 NaCl-structure TM nitride thin films also have

wide single-phase fields, which support large vacancy con-

centrations on the anion sublattice, resulting in the case of

TiN,22 in N/Ti ratios which range from 0.6 to 1.0,23 allowing

room-temperature resistivity of epitaxial TiN(001) layers to

be controllably varied from 13 to 190 lX cm (Ref. 24) and

the hardness from 20 to 30 GPa (Ref. 4) as N/Ti is decreased

from 1.0 to 0.6.

X-ray photoelectron spectroscopy (XPS) is often used to

provide not only compositional analyses of TM-nitride-

based pseudobinary, ternary, and higher-order thin film

alloys developed for specific applications, but also to acquirea)Electronic mail: grzgr@ifm.liu.se

05E101-1 J. Vac. Sci. Technol. A 33(5), Sep/Oct 2015 0734-2101/2015/33(5)/05E101/9/$30.00 VC 2015 American Vacuum Society 05E101-1

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.236.172.146 On: Mon, 30 Mar 2015 14:51:57

detailed information regarding elemental bonding configura-

tions. TM nitride films are typically grown by magnetron

sputter deposition in vacuum systems that do not contain in-situ XPS capability; the films are then air-exposed prior to

inserting them into a stand-alone XPS system. Ion etching is

used to remove oxygen and other adventitious surface con-

tamination prior to analysis. However, the etching process

can lead to preferential elemental sputter ejection, recoil

implantation, and structural disorder, all of which render

quantitative compositional and chemical analyses extremely

challenging.25

In order to circumvent these problems, capping layers have

been used to protect the sample surface from air exposure dur-

ing transport to the XPS instrument. For example, Kramer

et al. passivated surfaces of metastable (III–IV)1�x(IV2)x

semiconducting films with As, which was then desorbed

in-situ prior to XPS analysis.26,27 Thin capping layers

(�10 nm) were also used to allow direct XPS verification of

barrier integrity and check for cap-layer/sample-surface

reactions.28

Here, we investigate the effectiveness of metal capping

layers for nondestructive high-resolution XPS analyses of

ceramic thin films. We use, as a model materials system,

polycrystalline 200-nm-thick NaCl-crystal structure TiN

layers grown on Si(001) substrates at 400 �C by reactive

magnetron sputter deposition in mixed N2/Ar atmospheres.

The cap layer thickness is optimized such that high-quality

XPS spectra of the underlying TiN surface can be obtained

without significant attenuation. We select Al as the cap layer

for the following reasons: (1) the Al native oxide, <2 nm

thick at room temperature, is stable against spallation;5 (2)

the Al-N heat of formation (Df H0AlN¼�3.3 eV/atom) is

lower than that of Ti-N (Df H0TiN¼�3.5 eV/atom),29 thus

minimizing interfacial reactions; (3) and Al core-level peaks

do not overlap with the primary Ti and N signals. We show

that Al layers with thickness dAl¼ 1.5 nm form a dense

continuous, oxidized barrier that protects the TiN underlayer

and allows for acquisition of high-resolution Ti 2p and N 1s

core-level spectra, with clear pronounced satellite fea-

tures,30–33 which are in excellent agreement with those

obtained from epitaxial TiN layers grown in-situ in an XPS

system.34 O 1s spectra reveal no evidence for Ti–O bonding.

High-resolution Al 2p scans from 1.5-nm-Al/TiN/Si(001)

samples indicate that the entire cap layer is oxidized with no

Al/TiN interfacial reactions. In addition, XPS-determined

N/Ti compositional ratios, obtained based upon Ti 2p and N

1s peak areas, agree very well with the results of Rutherford

backscattering spectroscopy (RBS) and time-of-flight elastic

recoil detection analyses (ToF-ERDA).

II. EXPERIMENTAL PROCEDURE

Polycrystalline TiN/Si(001) layers, as well as Al cap

layers, are grown in a CemeCon CC800/9 magnetron sput-

tering system. The targets are cast rectangular 8.8� 50 cm2

Ti and Al plates (99.99% pure). Shutters are used to protect

one target while sputter etching the other, immediately prior

to film growth, in order to avoid cross-contamination.

Si(001) substrates, 1.5� 1 cm2, are cleaned sequentially in

acetone and isopropyl alcohol and mounted on a rotary

substrate table at a distance of six cm from the target. The

system is degassed prior to deposition using a two-step heat-

ing cycle: 1 h at 500 �C followed by 1 h during which the

temperature is slowly decreased to 400 �C, the deposition

temperature Ts. TiN layers, 200-nm thick, are grown in

mixed Ar/N2 atmospheres at a total pressure Ptot¼ 3 mTorr

(0.4 Pa). Ar and N2 flow rates are fAr¼ 350 cm3/min and

fN2¼ 50 cm3/min.

The Ti target is operated in high-power pulsed magnetron

sputtering mode at an average power of 1300 W, a pulsing

frequency of 1000 Hz, and a duty cycle of 20%. A substrate

bias voltage Vs¼ 60 V is applied in synchronous with the

target pulses.35–39 Following TiN deposition, fN2 is set to

zero while fAr is increased to 400 cm3/min to maintain Ptot

constant; the Al target is sputter-cleaned for 60 s with both

target shutters closed, and the TiN/Si(001) samples are

rotated in front of the Al target, which is operated at 0.3 kW

dc power for cap-layer deposition. Al overlayers are depos-

ited with thicknesses ranging from 1.5 to 25 nm, based upon

deposition rate calibrations. For reference, we also deposit

2-lm-thick Al films on TiN/Si(001) samples.

RBS analyses, using a 2.0 MeV 4Heþ probe beam inci-

dent at 10� with respect to the surface normal and detected at

a 172� scattering angle, as well as ToF-ERDA measurements

employing a 36 MeV 127I8þ probe beam incident at 67.5�

with recoils detected at 45�, show that the TiN films are

slightly understoichiometric with N/Ti¼ 0.96 6 0.01.

Film thicknesses determined from cross-sectional scanning

electron microscopy analyses in a LEO 1550 instrument are

in good agreement with deposition-rate calibrations.

X-ray diffraction h-2h scans and pole figure measure-

ments show that the TiN films are single-phase with the

cubic B1 NaCl structure. The layers are polycrystalline with

random in-plane orientation and no strong out-of-plane

orientation.

XPS spectra are acquired from air-exposed TiN and

Al/TiN films in a Kratos Analytical instrument, with a base

pressure of 1.1� 10�9 Torr (1.5� 10�7 Pa), using monochro-

matic Al Ka radiation (h�¼ 1486.6 eV) with the x-ray anode

operated at 225 W. The signal is detected orthogonal to the

sample surface. The Fermi edge cut-off is set, using sputter-

etched clean Ag foil, to an accuracy of better than 60.05 eV

and the position of the Ag 3d5/2 core-level peak is verified to

be 368.30 eV.40 All core-level (narrow energy range) spectra

are acquired with a pass energy Epass¼ 20 eV. For the Ag

3d5/2 reference peak, this results in a full-width at half maxi-

mum FWHM peak intensity of 0.55 eV. For survey (wide

energy range) scans, Epass¼ 160 eV resulting in a Ag 3d5/2

FWHM of 2.00 eV. Quantification is performed using

CASAXPS software (version 2.3.16), based upon peak areas

from narrow energy range scans with the Shirley-type back-

ground removed.41 Elemental sensitivity factors, corrected

for (1) the energy-dependent transmission function of the

spectrometer, and (2) the effect of kinetic-energy-dependent

electron mean free paths, are supplied by Kratos Analytical

Ltd.42

05E101-2 Greczynski et al.: Al capping layers for nondestructive XPS analyses 05E101-2

J. Vac. Sci. Technol. A, Vol. 33, No. 5, Sep/Oct 2015

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.236.172.146 On: Mon, 30 Mar 2015 14:51:57

Al-capped and uncapped TiN/Si(001) films are exposed

to air for �10 min during transport from the deposition

chamber to the XPS system where they are analyzed without

further processing. A second set of uncapped TiN samples is

subjected to sputter-etching in the XPS instrument prior to

analysis using an Ar ion beam with (a) an ion energy

EArþ ¼ 0.5 keV and a beam incidence angle of w¼ 70� rela-

tive to the surface normal; (b) EArþ ¼ 4 keV with w¼ 70�;(c) EArþ ¼ 4 keV, w¼ 45�; and (d) EArþ ¼ 4 keV, w¼ 0�. In

all cases, XPS spectra are obtained from a 0.3� 0.7 mm2

area at the center of the sputter-etched region after removal

of �10 nm.

Transport of ions in matter (TRIM),43 a Monte Carlo

program included in the stopping power and range of ions in

matter (SRIM) software package,44 is used to estimate

primary-ion and recoil projected ranges in TiN due to Ar ion

irradiation during sputter etching.

III. RESULTS

A. XPS analyses of Ar1-ion-etched air-exposedTiN/Si(001)

Typical Ti 2p and N 1s core-level spectra acquired from

sputter-etched uncapped air-exposed TiN surfaces are shown

in Figs. 1(a) and 1(b) for the four sets of EArþ=w etching con-

ditions. The Ti 2p core-level spectra consist of a spin-orbit

split doublet with Ti 2p3/2 and Ti 2p1/2 components at 455.2

and 461.1 eV, respectively. Both Ti 2p peaks exhibit satellite

features on the high binding-energy (BE) side, shifted

�2.7 eV above the primary peaks, in agreement with

previous XPS analyses of polycrystalline TiN layers grown

in-situ in an XPS system.45,46 To facilitate comparison, the

intensities of Ti 2p spectra are normalized to those of the

highest intensity features (Ti 2p3/2 at 455.2 eV, after subtrac-

tion of the low-BE background) for each spectrum. The

same scale factors are then used to normalize the corre-

sponding N 1s spectra in Fig. 1(b). The relative intensities of

the satellite peaks [see inset in Fig. 1(a)] are highest after

sputter etching with EArþ ¼ 0.5 keV and w¼ 70�; they

decrease in intensity upon increasing EArþ to 4 keV (at

w¼ 70�); and decrease even further as w is lowered to 45�

and 0�, while maintaining EArþ at 4 keV. The reduction in

the satellite feature intensity due to ion etching is accompa-

nied by increasing background levels on the high BE side.

The origin of Ti 2p satellite features from TiNx films with

x> 0.75 (Ref. 30) is widely discussed in the literature. Two

primary interpretations have been proposed including a

decrease in the screening probability of the core-hole created

during photoionization by Ti 3d electrons,30,32,45 and t1g !2t2g intraband transitions between occupied and unoccupied

electron states near the Fermi level (shake-up events).31,47

The intensity of the Ti 2p satellite features has also been

shown to be sensitive to changes induced by Ar ion bom-

bardment (residual point-defect creation, grain refinement,

atomic mixing, Ar trapping in interstitial sites, and N loss

due to preferential resputtering of lighter elements) as dem-

onstrated by Haasch et al.48 for epitaxial TiN/MgO(001)

films grown in-situ, with no air exposure, in an XPS system

and then ion-etched with a 3 keV Ar ion beam incident

at w¼ 40�. Thus, the changes in Ti 2p spectra shown in Fig.

1(a) are indicative of ion-irradiation-induced compositional

and structural modifications, which increase with increasing

Arþ ion penetration depth n.

The N 1s core-level peak at 397.4 eV, Fig. 1(b), is also

affected by ion irradiation. The peak intensity decreases,

accompanied by peak broadening toward the lower BE side,

with increasing ion energy EArþ and decreasing ion inci-

dence angle w. The satellite feature at �400.2 eV has a lower

intensity than the Ti 2p satellites [see Fig. 1(a)], but exhibits

a similar shift to higher BE with respect to the N 1s peak and

decreases in intensity with increasing EArþ and decreasing w,

i.e., increasing n. In addition, the XPS-determined N/Ti ratio

decreases from 0.74 6 0.03 with EArþ ¼ 0.5 keV and

w¼ 70�, to 0.72 6 0.03, 0.70 6 0.03, and 0.68 6 0.03 with

EArþ ¼ 4 keV and w¼ 70�, 45�, and 0�, indicating preferen-

tial N loss, in agreement with previous reports.48

The above results clearly illustrate issues associated with

XPS analyses following ion etching of air-exposed TiN

surfaces. Both Ti 2p and N 1s core-level spectra are sensitive

FIG. 1. (Color online) (a) XPS Ti 2p and (b) N 1s core-level spectra acquired

from sputter-etched uncapped air-exposed polycrystalline TiN/Si(001)

surfaces. The etching conditions are EArþ ¼ 0.5 keV/w¼ 70�, 4 keV/70�,4 keV/45�, and 4 keV/0�.

05E101-3 Greczynski et al.: Al capping layers for nondestructive XPS analyses 05E101-3

JVST A - Vacuum, Surfaces, and Films

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.236.172.146 On: Mon, 30 Mar 2015 14:51:57

to increased residual ion-irradiation-induced damage. The

effects are least visible for the lowest EArþ and highest w val-

ues used in these experiments. However, even for this case,

the uncapped TiN samples exhibit significant preferential N

loss due to resputtering.

B. XPS analyses of Al-capped TiN/Si(001)

Figures 2(a)–2(d) are typical SEM plan-view images of

Al/TiN/Si(001) samples with Al cap layer thicknesses dAl

ranging from 1.5 to 25 nm. The surface of the sample with

the thinnest capping layer is relatively featureless and

closely resembles that of the uncapped TiN film (not shown).

With increasing dAl, the average feature size increases from

<10 nm with dAl¼ 1.5 nm to 20 6 10, 45 6 15, and

90 6 20 nm with dAl¼ 6.0, 13.5, and 25.0 nm.

Figures 3(a)–3(d) are Ti 2p, N 1s, Al 2p, and O 1s core-

level spectra acquired from a series of TiN/Si(001) samples,

which were either uncapped or capped with Al layers with

thicknesses dAl from 1.5 to 25 nm prior to air exposure.

Samples used for this set of spectra are analyzed as-received

and not Arþ-ion etched.

The core level spectra from TiN/Si(001) samples with no

Al overlayer exhibit pronounced effects of air exposure; the

Ti 2p spectrum in Fig. 3(a) shows that Ti is present in three

chemical states: TiN, TiOxNy, and Ti-oxide giving rise to Ti

2p3/2 peaks at 455.2, 456.7, and 458.2 eV, respectively.49,50

The corresponding N 1s spectrum [Fig. 3(b)] also contains

three peaks: TiN at 397.4 eV,50,51 TiOxNy at 396.1 eV,49 and

a low-intensity TiN satellite at �400.2 eV, while the O 1s

spectrum [Fig. 3(d)] consists of two peaks at 529.9 and

531.4 eV attributed to oxygen in Ti-oxide and TiOxNy bond-

ing configurations, respectively.49,50 Thus, the Ti 2p, N 1s,

and O 1s spectra are consistent in indicating the presence of

Ti-oxide and TiOxNy due to air exposure.

In distinct contrast, spectra recorded from TiN films

with Al cap layers exhibit no indication of Ti-oxide or

Ti-oxynitride formation. Ti 2p and N 1s core-level signals,

Figs. 3(a) and 3(b), resemble those acquired from epitaxial

TiN/MgO(001) layers grown in-situ in an XPS system, and

analyzed with no air-exposure or Arþ ion etching.48 The O

1s spectrum, Fig. 3(d), consists of only a single peak at

532.0 eV, shifted toward higher BE with respect to the

uncapped sample, corresponding to Al–O bonding in

Al-oxide. The Al 2p spectra in Fig. 3(c) reveals that the

1.5-nm-thick Al cap layers are fully oxidized with an oxide

peak at 75.1 eV and no observable contribution due to metal-

lic Al.40

With increasing Al capping layer thickness dAl, the Ti 2p

and N 1s peak intensities decrease due to inelastic electron

scattering in the overlayer, as discussed further in Sec. IV.

There are no detectable Ti 2p or N 1s peak shifts or shape

changes. The O 1s peak increases and shifts slightly from

532.0 to 532.4 eV, due to charging in the Al-oxide layer,

characteristic of native Al oxide formation as observed

for the 2-lm-thick Al reference layer. The Al 2p spectra in

Fig. 3(c) exhibit only a single broad peak at 75.1 eV corre-

sponding to native Al oxide.50 The Al 2p spectra from sam-

ples with dAl� 6.0 nm contain, in addition to the Al–O peak,

a metallic Al peak at 72.9 eV,40 which increases in intensity

with increasing Al layer thickness. For samples with

dAl¼ 25 nm, the Al 2p spectra is essentially identical to that

acquired from the 2-lm-thick Al reference layer. The metal-

lic peak has a much lower FWHM allowing spin-orbit split-

ting (DE¼ 0.4 eV) to be resolved with Al 2p3/2¼ 72.8 eV

and Al 2p1/2¼ 73.2 eV. The BE of the Al-oxide peak

increases slightly to 75.5 6 0.1 eV for samples with cap layer

thickness dAl¼ 13.5 and 25.0 nm. There is no evidence of

AlN formation, which would lead to a peak at 74.2 eV,33 in

the Al 2p spectra.

The Ti 2p and N 1s spectra in Figs. 4(a) and 4(b),

from uncapped TiN/Si(001) samples ion-etched with

EArþ ¼ 0.5 keV and w¼ 70� (the conditions resulting in the

FIG. 2. SEM plan-view images of polycrystalline Al/TiN/Si(001) films with Al cap layer thicknesses dAl of: (a) 1.5, (b) 6.0, (c) 13.5, and (d) 25.0 nm.

05E101-4 Greczynski et al.: Al capping layers for nondestructive XPS analyses 05E101-4

J. Vac. Sci. Technol. A, Vol. 33, No. 5, Sep/Oct 2015

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.236.172.146 On: Mon, 30 Mar 2015 14:51:57

least sample damage) are directly compared with those

obtained from air-exposed, but unetched, Al/TiN/Si(001)

samples with 1.5-nm-thick Al cap layers. To facilitate com-

parison, Ti 2p3/2 Ti-N bonding peaks at 455.2 eV are normal-

ized in the manner described earlier. The same scaling

factors are then used to normalize the corresponding N 1s

spectra in order to illustrate differences in XPS-determined

N/Ti ratios.

The Ti 2p satellite intensity obtained from the Al-capped

sample is higher than for all Arþ ion-etched surfaces and

comparable to that acquired from epitaxial TiN/MgO(001)

films grown in-situ in an XPS system with no air exposure.34

The increase in the background intensities on the high-BE

sides of the Ti 2p peaks from the capped sample in Fig. 4(a),

due to inelastic electron scattering in the Al overlayer, is

very small and does not degrade the primary peak signals.

The intensity of the N 1s Ti-N peak obtained from the

1.5-nm Al/TiN/Si(001) sample, Fig. 4(b), is much higher

than that of all uncapped TiN samples, even those subjected

to the mildest etching conditions (EArþ ¼ 0.5 keV and

w¼ 70�) for which the XPS-determined N/Ti ratio is

0.74 6 0.03. The N/Ti ratio obtained from the Al-capped

TiN sample is 0.98 6 0.03, in good agreement with RBS and

ToF-ERDA results of 0.96 6 0.01. The inset in Fig. 4(b)

shows normalized N 1s spectra for uncapped TiN/Si(001)

samples ion-etched with EArþ ¼ 0.5 keV/w¼ 70� together

with air-exposed, but unetched, Al/TiN/Si(001) samples

with 1.5-nm-thick Al cap layers. The shape of the N 1s spec-

tra acquired from TiN samples with Al capping layers is

very similar to that of the air-exposed uncapped TiN/Si(001)

film ion etched with EArþ ¼ 0.5 keV and w¼ 70�, with the

higher BE N 1s satellite peak at �400.2 eV clearly resolved.

There is an additional low-intensity, low-BE component in

the N 1s spectra, Fig. 4(b), obtained from Al/TiN/Si(001)

due to organic contamination during air exposure. This

assignment is supported by the fact that the intensity of this

feature increases with increasing air exposure time.

C. Stability of Al cap layers versus air-exposure time

The stability of Al cap layers as a function of air-

exposure time determines the maximum time allowed for

sample transfer between deposition and XPS systems. Here,

we focus on the thinnest Al cap layer, dAl¼ 1.5 nm, which

provides the least XPS signal attenuation. Al/TiN/Si(001)

multilayers are stored in laboratory air, 23 �C and 40% rela-

tive humidity, for times ranging from 10 to 100 000 min.

Figures 5(a)–5(e) show C 1s, Al 2p, O 1s, Ti 2p, and N 1s

core-level spectra recorded after t¼ 10, 100, 1000, 10 000,

and 100 000 min of air exposure. The most pronounced

change with t is observed in C 1s spectra, which contain

three peaks centered at 282.1, 285.6, and 289.9 eV, corre-

sponding to the chemical bonding states C–Al, C–C, and

C¼O (and/or O–C¼O).50,52 The intensities of the two

higher-energy peaks increase with storage time indicating

continuous accumulation of adventitious carbon on the sur-

face, while the lower-energy peak, due to interactions at the

C/Al interface, remains unchanged. The Al 2p spectra exhib-

its a single Al-oxide peak at 75.1 eV, with no metallic fea-

ture, which does not change in intensity or shape as a

function of air exposure time, indicating that 1.5-nm-thick

FIG. 3. (Color online) XPS (a) Ti 2p, (b) N 1s, (c) Al 2p, and (d) O 1s core-level spectra acquired from a series of air-exposed polycrystalline TiN/Si(001)

films, which were uncapped and capped with Al overlayers of thickness dAl¼ 1.5, 6.0, 13.5, and 25.0 nm prior to air exposure. The samples were analyzed as

received with no Ar þ ion etching.

05E101-5 Greczynski et al.: Al capping layers for nondestructive XPS analyses 05E101-5

JVST A - Vacuum, Surfaces, and Films

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.236.172.146 On: Mon, 30 Mar 2015 14:51:57

cap layers form a complete stable oxide in �10 min of air

exposure. There is no indication of interfacial Al/TiN inter-

actions, even for the longest storage times. The only observ-

able change in all other core level signals, Figs. 5(c)–5(e), is

a small decrease in signal intensity with storage time due to

the increased thickness of the carbon contamination layer;

there is no evidence in time-dependent O 1s spectra for the

formation of Ti oxide or oxynitride.

IV. DISCUSSION

Results shown in Fig. 1(a) for ion-etched uncapped

air-exposed polycrystalline TiN/Si(001) layers with no

strong preferred orientation indicate that the Ti 2p satellite

intensity decreases with increasing ion energy and ion

penetration depth n. The corresponding set of N 1s spectra in

Fig. 1(b) reveals that upon Ar ion bombardment, both the

primary and satellite peaks exhibit a decrease in intensity

and the primary peak broadens on the low-BE side. The

XPS-determined N/Ti ratios for all ion-etched air-exposed

samples are significantly lower than the value obtained from

RBS and ERDA analyses, even for the mildest set of ion

etching conditions (EArþ ¼ 0.5 keV and w¼ 70�) and

decreases further with increasing ion penetration depth n.Similar effects were reported for epitaxial TiN/MgO(001)

films grown and ion-etched in-situ, with no air exposure, in

an XPS system.48

The ion penetration depth n, which increases with

increasing ion energy EArþ and decreasing ion incidence

angle w, is characterized by the effective depth of collision

cascade events, which can be estimated from Monte-Carlo

based TRIM simulations of ion/surface interactions, and

corresponds to the average TiN primary recoil projected

range accounting for straggle. For Ti recoils, nTi¼ 1.2 nm

with EArþ ¼ 0.5 keV and w¼ 70�, and increases to 2.6, 4.0,

and 5.3 nm with EArþ ¼ 4 keV and w¼ 70�, 45�, and 0�. For

N recoils, nN increases from 0.6 to 2.3, 4.0, and 5.3 nm for

the same ion bombardment conditions. These length scales

are comparable to typical XPS probing depths, which are

controlled by electron inelastic mean free paths k. The frac-

tion of the XPS signal intensity Id originating from a surface

layer of thickness d is given by ½1� exp ð�d=kÞ�. With

k¼ 2.2 nm (for electrons in TiN with energy E¼ 1300 eV)

(Ref. 53) and d� nTi (approximate width of ion-altered

surface layer), the percentage contribution to the Ti and N

core level XPS signal originating from the Arþ ion beam

modified surface layer of thickness d is �40% with

EArþ ¼ 0.5 keV and w¼ 70�, and increases to �70, �85,

and �90% with EArþ ¼ 4 keV and w¼ 70�, 45�, and 0�.Thus, a significant fraction of the XPS signal arises from an

ion-irradiation-altered TiN surface layer even for the mildest

set of ion etching conditions used in these experiments and

increases to essentially the entire sampling depth in the

case of etching with EArþ ¼ 4 keV/w¼ 0�. A serious ion-

irradiation effect is the loss of N due to preferential resput-

tering. For the mildest ion etch (EArþ ¼ 0.5 keV/w¼ 70�),i.e., lowest n, the XPS-determined N/Ti ratio is 0.74 6 0.03,

much lower than the actual value, 0.96 6 0.01, obtained

from RBS and ERDA. XPS N/Ti ratios decrease further

to 0.68 6 0.03 as EArþ is increased to 4 keV and w decreased

to 0�.The use of Al capping layers with thicknesses of

1.5–25.0 nm prevents TiN oxidation during sample air expo-

sure as evident from the Ti 2p, N 1s, and O 1s spectra shown

in Figs. 3(a), 3(b), and 3(d). Clean TiN spectral features are

preserved, in particular, the Ti 2p and N 1s satellite features

are intact, unlike the corresponding spectra from ion-etched

air-exposed samples (see Fig. 1). There is no evidence in

either the Al 2p or N 1s spectra indicating interfacial Al/TiN

reactions. This is consistent with TiN having a larger heat of

formation (Df H0TiN¼�3.5 eV/atom) than AlN (Df H

0A1N

¼�3.3 eV/atom),29 as well as with earlier studies which

show that the Al/TiN interface is stable up to 500 �C;54–56

i.e., to a significantly higher temperature than used during Al

deposition in the present experiments.

TiN oxidation during air exposure is prevented even by

the thinnest Al capping layer, dAl¼ 1.5 nm, indicating that

the overlayer is continuous, in agreement with SEM images

[see, for example, Fig. 2(a)] showing a smooth surface. The

Al 2p core-level signal in Fig. 3(c) reveals that 1.5-nm-thick

Al cap layers are completely oxidized, with no evidence of a

FIG. 4. (Color online) XPS (a) Ti 2p and (b) N 1s spectra from uncapped air-

exposed for �10 min polycrystalline TiN/Si(001) samples ion-etched with

EArþ ¼ 0.5 keV and w¼ 70� compared to an air-exposed, but unetched,

Al/TiN/Si(001) sample with a 1.5-nm-thick Al cap layer. To facilitate

comparison, each Ti 2p spectrum is normalized to the highest intensity peak

(Ti 2p3/2 peaks at 455.2 eV); the corresponding N 1s spectra are scaled with

the same factors. In order to highlight differences in peak shape, normalized

N 1s spectra are shown in the inset in (b).

05E101-6 Greczynski et al.: Al capping layers for nondestructive XPS analyses 05E101-6

J. Vac. Sci. Technol. A, Vol. 33, No. 5, Sep/Oct 2015

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.236.172.146 On: Mon, 30 Mar 2015 14:51:57

metallic Al peak. We estimate the thickness of the native Al

oxide layer, based upon the intensity ratio of the Al 2p oxide

to metal core level peaks57 acquired from the air-exposed

2-lm-thick Al film, to be 2.1 nm. This is in very good

agreement with the value, 2.2 nm, we obtain by assuming

that the Al and Al2O3 layers are fully dense such that

tox ¼ tAlðqAlMoxÞ=ðqoxMAlNAlÞ, in which t is the layer thick-

ness, q is density, M is the mass in amu, and NAl¼ 2 is the

number of Al atoms per oxide molecule.

Ti 2p and N 1s peak intensities from Al/TiN/Si(001) sam-

ples decrease with increasing Al capping layer thickness dAl,

due to inelastic electron scattering in the cap layer.

However, the signal does not decay exponentially with

increasing dAl, as would be expected in the case of attenua-

tion by a continuous overlayer for which core-level intensity

drops as � exp ð�dAl=kAlÞ, in which kAl is the inelastic

mean free path of Ti 2p and N 1s electrons in Al. This

can be interpreted with the help of SEM images in Figs.

2(b)–2(d), which reveal significant surface roughness in

samples with 6.0� dAl� 25.0 nm. Moreover, the roughness

increases with increasing cap layer thickness, thus the

average near-surface film density decreases. This accounts

for the less-than-exponential decrease in Ti 2p and N 1s

core-level intensities with dAl.

Ion-etched air-exposed TiN/Si(001) samples suffer from

severe N loss resulting from preferential resputtering; the

XPS-determined N/Ti values range from 0.74 6 0.03 for the

mildest etching conditions (EArþ ¼ 0.5 keV and w¼ 70�) and

decrease continuously to 0.68 6 0.03 for the most severe

Arþ ion etch (EArþ ¼ 4 keV and w¼ 0�). In contrast, XPS

N/Ti ratio obtained from the Al-capped TiN samples is sig-

nificantly higher, 0.98 6 0.03, in very good agreement with

RBS and ToF-ERDA results, 0.96 6 0.01.

V. CONCLUSIONS

We show that thin metal layers provide effective barriers

to sample oxidation and contamination during air exposure

and allow subsequent quantitative XPS analyses in which

FIG. 5. (Color online) XPS (a) C 1s, (b) Al 2p, (c) O 1s, (d) Ti 2p, and (e) N 1s core-level spectra from air-exposed Al/TiN/Si(001) samples for which the Al

cap layer thicknesses are dAl¼ 1.5 nm. The spectra are recorded following t¼ 10, 100, 1000, 10 000, and 100 000 min of air exposure.

05E101-7 Greczynski et al.: Al capping layers for nondestructive XPS analyses 05E101-7

JVST A - Vacuum, Surfaces, and Films

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.236.172.146 On: Mon, 30 Mar 2015 14:51:57

ion etching is not required. Here, we use polycrystalline

TiN/Si(001) thin films as a model system and demonstrate

that deposition of 1.5-nm-thick Al layers prior to air expo-

sure allows nondestructive acquisition of high-resolution

core-level spectra representative of clean samples, and

hence, provide correct bonding assignments. The Ti 2p and

N 1s satellite features, which are sensitive to ion bombard-

ment, exhibit high intensities comparable to those obtained

from single-crystal TiN/MgO(001) films grown and ana-

lyzed in-situ in a ultrahigh-vacuum XPS system; line shapes

and peak energies are also in excellent agreement. There is

no indication of reaction between the Al cap layers and

the underlying TiN films, or for the formation of Ti oxide.

XPS-determined N/Ti concentrations acquired from Al/TiN/

Si(001) samples agree very well with results obtained by

Rutherford backscattering and elastic recoil analyses. In con-

trast, XPS-determined N/Ti ratios of air-exposed uncapped

TiN/Si(001) samples subjected to ion etching to remove

oxides and adventitious contamination exhibit clear evidence

of strong preferential N loss, which increases with increasing

Ar ion energy and decreasing incidence angle.

Al 2p core-level spectra from air-exposed Al/TiN/

Si(001) samples reveal that 1.5-nm-thick Al cap layers

are immediately fully oxidized with no evidence of a

metallic Al XPS peak. However, XPS cap layer spectra

exhibit increasingly strong metallic Al peaks as dAl is

increased from 6 to 25 nm. For all samples, irrespective

of dAl, there is no Al 2p or N 1s spectral indication of

AlNx formation. Moreover, the intensities and shapes of

the Ti 2p and N 1s core level signals from TiN do not

change following long-term (up to 70 days) sample expo-

sure to ambient conditions prior to analysis showing that

the thin Al cap layers provide stable surface passivation

without spallation.

The metal cap-layer strategy for eliminating ion etching

of air-exposed samples prior to quantitative XPS analyses,

demonstrated here for TiN/Si(001) thin films with an Al cap,

can be applied to other material systems. The primary

requirements for the choice of cap layer material are: (1) it

should form a dense, continuous, and stable oxide (no spalla-

tion), (2) the cap should be thin to avoid significant signal

attenuation from the underlying sample, (3) there should be

no cap/sample interfacial reaction, and (4) core-level peaks

from the cap layer should not overlap with those from the

sample.

ACKNOWLEDGMENTS

Financial support from the European Research Council

(ERC) through an Advanced Grant No. 227754, the VINN

Excellence Center Functional Nanoscale Materials(FunMat) Grant No. 2005-02666, the Knut and Alice

Wallenberg Foundation Grant No. 2011.0143, the Swedish

Government Strategic Faculty Grant in Materials Science to

Link€oping University (SFO Mat-LiU AFM), and Swedish

Research Council (VR) Project Grant No. 2014-5790 are

gratefully acknowledged. Jens Jensen is acknowledged for

carrying out the ERDA and RBS analyses.

1A. H€orling, L. Hultman, M. Od�en, J. Sj€ol�en, and L. Karlsson, Surf. Coat.

Technol. 191, 384 (2005).2P. H. Mayrhofer, C. Mitterer, L. Hultman, and H. Clemens, Prog. Mater.

Sci. 51, 1032 (2006).3H. Ljungcrantz, M. Od�en, L. Hultman, J. E. Greene, and J.-E. Sundgren,

J. Appl. Phys. 80, 6725 (1996).4C.-S. Shin, D. Gall, N. Hellgren, J. Patscheider, I. Petrov, and J. E.

Greene, J. Appl. Phys. 93, 6025 (2003).5D. McIntyre, J. E. Greene, G. Hakansson, J.-E. Sundgren, and W.-D.

M€unz, J. Appl. Phys. 67, 1542 (1990).6L. A. Donohue, I. J. Smith, W.-D. M€unz, I. Petrov, and J. E. Greene, Surf.

Coat. Technol. 94/95, 226 (1997).7A. Ingason, F. Magnus, J. S. Agustsson, S. Olafsson, and J. T.

Gudmundsson, Thin Solid Films 517, 6731 (2009).8D. Gall, I. Petrov, and J. E. Greene, J. Appl. Phys. 89, 401 (2001).9A. B. Mei, A. Rockett, L. Hultman, I. Petrov, and J. E. Greene, J. Appl.

Phys. 114, 193708 (2013).10C.-S. Shin, D. Gall, Y.-W. Kim, P. Desjardins, I. Petrov, J. E. Greene, M.

Od�en, and L. Hultman, J. Appl. Phys. 90, 2879 (2001).11H.-S. Seo, T.-Y. Lee, I. Petrov, J. E. Greene, and D. Gall, J. Appl. Phys.

97, 083521 (2005).12O. Knotek, M. B€ohmer, and T. Leyendecker, J. Vac. Sci. Technol. A 4,

2695 (1986).13T. Leyendecker, O. Lemmer, S. Esser, and J. Ebberink, Surf. Coat.

Technol. 48, 175 (1991).14J. M. Molarius, A. S. Korhonen, E. Harju, and R. Lappalainen, Surf. Coat.

Technol. 33, 117 (1987).15V. R. Parameswaran, J.-P. Immarigeon, and D. Nagy, Surf. Coat. Technol.

52, 251 (1992).16M.-A. Nicolet, Thin Solid Films 52, 415 (1978).17I. Petrov, E. Mojab, F. Adibi, J. E. Greene, L. Hultman, and J.-E.

Sundgren, J. Vac. Sci. Technol. A 11, 11 (1993).18J. S. Chun, I. Petrov, and J. E. Greene, J. Appl. Phys. 86, 3633

(1999).19J. S. Chun, P. Desjardins, I. Petrov, J. E. Greene, C. Lavoie, and C.

Cabral, Jr., Thin Solid Films 391, 69 (2001).20J. S. Chun, P. Desjardins, C. Lavoie, C.-S. Shin, C. Cabral, Jr., I. Petrov,

and J. E. Greene, J. Appl. Phys. 89, 7841 (2001).21B. Subramanian, C. V. Muraleedharan, R. Ananthakumar, and M.

Jayachandran, Surf. Coat. Technol. 205, 5014 (2011).22J.-E. Sundgren, B.-O. Johansson, A. Rockett, S. A. Barnett, and J. E.

Greene, in Physics and Chemistry of Protective Coatings, edited by J. E.

Greene, W. D. Sproul, and J. A. Thornton (American Institute of Physics,

New York, 1986), Ser. 149, p. 95.23T. Lee, K. Ohmori, C.-S. Shin, D. G. Cahill, I. Petrov, and J. E. Greene,

Phys. Rev. B 71, 144106 (2005).24C.-S. Shin, S. Rudenja, D. Gall, N. Hellgren, T.-Y. Lee, I. Petrov, and J.

E. Greene, J. Appl. Phys. 95, 356 (2004).25D. Gall, R. Haasch, N. Finnegan, T.-Y. Lee, C.-S. Shin, E. Sammann, J. E.

Greene, and I. Petrov, Surf. Sci. Spectra 7, 167 (2000).26B. Kramer, G. Tomasch, M. Ray, J. E. Greene, L. Salvati, and T. L. Barr,

J. Vac. Sci. Technol. A 6, 1572 (1988).27B. Kramer, G. Tomasch, J. E. Greene, L. Salvati, T. L. Barr, and M. Ray,

Phys. Rev. B: Condens. Matter Mater. Phys. 46, 1372 (1992).28L. Gan, R. D. Gomez, C. J. Powell, R. D. McMichael, P. J. Chen, and W.

F. Egelhoff, Jr., J. Appl. Phys. 93, 8731 (2003).29“National Institute of Standards and Technology (NIST) Chemistry

WebBook,” see http://webbook.nist.gov/chemistry/, accessed 18 December

2014.30L. Porte, L. Roux, and J. Hanus, Phys. Rev. B 28, 3214 (1983).31I. Strydom and S. Hofmann, J. Electron Spectrosc. Relat. Phenom. 56, 85

(1991).32J. Patscheider, N. Hellgren, R. T. Haasch, I. Petrov, and J. E. Greene,

Phys. Rev. B 83, 125124 (2011).33G. Greczynski, J. Jensen, J. E. Greene, I. Petrov, and L. Hultman, Surf.

Sci. Spectra 21, 35 (2014).34R. T. Haasch, T.-Y. Lee, D. Gall, J. E. Greene, and I. Petrov, Surf. Sci.

Spectra 7, 193 (2000).35G. Greczynski, J. Lu, M. Johansson, J. Jensen, I. Petrov, J. E. Greene, and

L. Hultman, Surf. Coat. Technol. 206, 4202 (2012).36G. Greczynski, J. Lu, M. Johansson, J. Jensen, I. Petrov, J. E. Greene, and

L. Hultman, Vacuum 86, 1036 (2012).37G. Greczynski et al., J. Vac. Sci. Technol. A 30, 061504 (2012).

05E101-8 Greczynski et al.: Al capping layers for nondestructive XPS analyses 05E101-8

J. Vac. Sci. Technol. A, Vol. 33, No. 5, Sep/Oct 2015

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.236.172.146 On: Mon, 30 Mar 2015 14:51:57

38G. Greczynski et al., Thin Solid Films 556, 87 (2014).39G. Greczynski, J. Lu, I. Petrov, J. E. Greene, S. Bolz, W. K€olker, Ch.

Schiffers, O. Lemmer, and L. Hultman, J. Vac. Sci. Technol. A 32,

041515 (2014).40J. F. Moulder,W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook of

X-ray Photoelectron Spectroscopy (Perkin-Elmer Corporation, Eden

Prairie, MN, 1992).41D. A. Shirley, Phys. Rev. B 5, 4709 (1972).42Kratos Analytical Ltd., Library filename: casaXPS_KratosAxis-

F1s.lib.43J. F. Ziegler, J. P. Biersack, and U. Littmark, The Stopping and Range of

Ions in Solids, Stopping and Ranges of Ions in Matter Vol. 1 (Pergamon,

New York, 1984).44“Particle interactions with matter,” www.srim.org, accessed 28 June 2013.45A. Arranz and C. Palacio, Surf. Sci. 600, 2510 (2006).46I. Bertoti, M. Mohai, J. L. Sullivan, and S. O. Saied, Surf. Interface Anal.

21, 467 (1994).47D. Jaeger and J. Patscheider, J. Electron Spectrosc. Relat. Phenom. 185,

523 (2012).48R. T. Haasch, T.-Y. Lee, D. Gall, J. E. Greene, and I. Petrov, Surf. Sci.

Spectra 7, 204 (2000).49P. Prieto and R. E. Kirby, J. Vac. Sci. Technol. A 13, 2819 (1995).50A. V. Naumkin, A. Kraut-Vass, S. W. Gaarenstroom, and C. J. Powell,

“National Institute of Standards and Technology (NIST), X-ray

photoelectron spectroscopy database,” version 4.1, http://srdata.nist.gov/

xps/, accessed 17 December 2014.51B. M. Biwer and S. L. Bernasek, J. Electron Spectrosc. Relat. Phenom. 40,

339 (1986).52See Appendix E in Surface Analysis by Auger and X-ray Photoelectron

Spectroscopy, edited by D. Briggs and J. T. Grant (IM Publications and

Surface Spectra Limited, UK, 2003).53The electron mean free path in TiN k¼ 2.2 nm is determined based upon a

separate set of experiments in which TiN/ZrN/Si(001) samples with TiN

thicknesses varying from 0 to 20 nm are analysed. k is estimated from the

attenuation of the Zr 3d core level signal (BE¼ 180 eV) excited with Al Kax-rays (h�¼ 1486.6 eV), resulting in an electron kinetic energy Ee� 1300

eV. Ee is lower for Ti 2p and N 1s electrons (1030 and 1080 eV, respectively)

and the corresponding mean free paths are somewhat shorter than 2.2 nm,

thus estimates of the percentage contributions to Ti 2p and N 1s core-level

XPS signals from the Ar þ ion beam altered surface layer are lower limits.54L. Hultman, S. Benhenda, G. Radnoczi, J.-E. Sundgren, J. E. Greene, and

I. Petrov, Thin Soild Films 215, 152 (1992).55I. Petrov, E. Mojab, R. C. Powell, J. E. Greene, L. Hultman, and J.-E.

Sundgren, Appl. Phys. Lett. 60, 2491 (1992).56J.-S. Chun, J. R. A. Carlsson, P. Desjardins, D. B. Bergstrom, I. Petrov, J.

E. Greene, C. Lavoie, C. Cabral, Jr., and L. Hultman, J. Vac. Sci. Technol.

A 19, 182 (2001).57B. R. Strohmeier, Surf. Interface Anal. 15, 51 (1990).

05E101-9 Greczynski et al.: Al capping layers for nondestructive XPS analyses 05E101-9

JVST A - Vacuum, Surfaces, and Films

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.236.172.146 On: Mon, 30 Mar 2015 14:51:57